tcp: md5: remove one indirection level in tcp_md5sig_pool

tcp_md5sig_pool is currently an 'array' (a percpu object) of pointers to
struct tcp_md5sig_pool. Only the pointers are NUMA aware, but objects
themselves are all allocated on a single node.

Remove this extra indirection to get proper percpu memory (NUMA aware)
and make code simpler.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
tcp: fix build error if !CONFIG_SYN_COOKIES

commit 946cedccbd7387 (tcp: Change possible SYN flooding messages)
added a build error if CONFIG_SYN_COOKIES=n

Reported-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>